(1) Field of the Invention
This invention relates to a hydraulic shock absorber of such a type that it is primarily used for a suspension device for an automobile, and a vibration damping force is generated by utilizing restriction resistance of fluid.
(2) Description of the Prior Art
The hydraulic shock absorber of the above-mentioned type generally includes a cylinder which is oil-tightly separated into a piston upper chamber and a piston lower chamber by a piston reciprocating in the cylinder. The piston is provided with a plurality of piston ports in a circle for communicating the piston upper chamber with the piston lower chamber, and is further provided with a plurality of return ports in a circle outside of the piston ports for communicating the piston upper chamber with the piston lower chamber. A sheet ring-like non-return valve is arranged above the piston and slidably fitted to a piston rod. The non-return valve is provided with valve holes opposed to the piston ports. A ring-like non-return valve stopper is arranged above the non-return valve with a small space defined between the valve stopper and the non-return valve, and is fixed to the piston rod. The non-return valve is biased against an upper surface (seal surface) of the piston by a spring interposed between the non-return valve and the valve stopper. Each inlet of the piston ports is communicated with each groove (arcuate grooves or ring groove) formed on the seal surface of the piston along a circle where the piston ports lie. If the piston ports are completely registered with the valve holes, this groove is not required to be provided. However, as the piston ports are offset from the valve holes upon installation of the hydraulic shock absorber, this groove is provided for purpose of communication of the piston ports and the valve holes irrespective of the offset of the former from the latter. When the piston is moved to the piston upper chamber to allow oil to flow from the piston upper chamber through the piston ports to the piston lower chamber, and at this time, there is created a restricting action against oil flow. Upon receiving impact, a vibration damping force against the impact is generated by the restricting action. During flowing of the oil from the piston upper chamber to the piston lower chamber, the return port is closed by the non-return valve to block communication of the oil. On the other hand, during flowing of the oil from the piston lower chamber, the non-return valve is pushed up from the seal surface of the piston to allow the oil to flow in the return port, and the piston ports are closed by a check valve provided under the piston ports. In this manner, it is designed that vibration damping characteristics in the upward and downward actions are different.
The piston is generally made of sintered materials so as to improve moldability, and a depth of the groove is small so as to secure a strength of the seal surface. Further, iron powder is scattered on the seal surface outside of the groove, and is deposited by burning. However, in the case that the depth of the groove is small, there is a possibility that the vibration damping force is varied according to a positional relation between the valve holes and the inlets of the piston ports upon installation of the shock absorber, and because of fluctuation in resistance of the oil flowing from the valve holes through the grooves to the inlets of the piston ports.